As a subclass of commercially available polymers, polyurethane foams have several properties whose advantages confer unique benefits on these products. Compared to many other polymeric foams, polyurethanes have almost immediate recovery when compressed, show excellent bonding to many types of synthetic and natural fibers, have a high index of insulation, and can be made with a very high percentage of open cells or a combination of open and closed cells. Many of the advantages of polyurethane foam are due to the tremendous versatility of the material. Polyurethane foams can be manufactured from the very soft to the very rigid by making only small, simple changes in the formulation. They may be made at temperatures lower than 25.degree.C. to over 100.degree.C. The polyurethane foams are by far one of the easiest to process. They can be poured by hand or machine, made in batch mode or made continuously and poured onto a conveyor belt to produce long buns, injected into molds, and even fabricated to produce an integral skin at the same time it produces the lower density foam core. This last type of product is also produced using the technique of reaction injection molding (RIM). Compared to other plastics, polyurethanes are non-brittle, much more resistant to abrasion, and exhibit good elastomeric memory. Polyurethanes find use in such diverse products as bushings, gaskets, washers, scraper blades, mattresses, furniture cushioning, car seats, headrests, shock absorbing pads, protective cushioning, protective packaging, insulation, filling and sealing cracks, shoe soles, window frames, automobile bumpers, dashboards, and appliance housings.
Part of the utility of polyurethane foams derives from their enormous diversity of properties resulting from a relatively limited number of reactants. Polyurethane foams are typically prepared by allowing a polyisocyanate to react with a combination of backbone polyols (e.g., polyols with molecular weights above about 500), curing agents (typically polyols with molecular weights below about 500), blowing agent(s), surfactant(s), catalyst(s), and possibly other additives such as fillers, pigments, softening agents, and flame retardants. The polyisocyanates may be chosen from polyisocyanate monomers, prepolymers, modified polyisocyanates such as trimerized polyisocyanates, carbodiimide-modified polyisocyanates, or from any other of the several types of variants. In all cases, a suitable blowing agent is used. Curing is the reaction of the terminal isocyanate groups with the active hydrogens of a polyfunctional compound so as to form high polymers through chain extension and, in some cases, crosslinking. In the prior art, polyols are almost exclusively used as the curing agents for MDI- and TDI-based foams. Where a triol or a high polyhydric alcohol is used crosslinking occurs to afford a nonlinear polymer. Components such as catalysts, pigments, surfactants, and blowing agents also may be present.
Although other polyfunctional chemicals, especially diamines, are theoretically suitable, with but a few exceptions none have achieved commercial importance as a curing agent and then not for flexible foams. Generally speaking, primary polyamines react with polyisocyanates, and especially MDI-based polyisocyanates, so quickly that they are not usable as curing agents (or additives) for flexible foam. The exception to this is the amines generated internally from the reaction of the small amount of water, used to "blow" the reaction, with the polyisocyanates. One reason that polyhydric alcohols generally have gained acceptance as curing agents for flexible foams is that their reaction with polyisocyanates is sufficiently fast to be convenient, but not so fast as to make it difficult to work with the resulting polymer. In producing foams it is desirable that the cream time be reasonably short, yet long enough for the material to be injected into molds or poured onto the conveyor. The material must also be fluid long enough for suitable foaming to occur (rise time).
One difficulty with the flexible foams of the prior art is their tendency to discolor, especially to yellow, accompanying oxidation or photochemically-induced degradation. Consequently, most flexible foams have incorporated antioxidants and/or ultraviolet stabilizers in their formulation, leading to increased costs, often with limited or uncertain benefits. Yellowing is most often associated with aromatic curing agents, consequently there is a great deal of incentive to prepare foams using completely aliphatic curing agents. We have found that the class of bis-(N-alkylaminocyclohexyl)methanes as curing agents affords flexible foams having quite desirable properties with respect to outstanding resistance to yellowing.